Greek letter taken up? Cheers! (5)
I believe the answer is:
delta
'greek letter' is the definition.
(letter from the Greek alphabet)
'taken up? cheers' is the wordplay.
'taken' becomes 'led' (to lead someone somewhere is to take them there).
'up?' shows that the letters should be reversed in order (in down clue: letters go upwards).
'cheers' becomes 'ta' (both can informally mean thank you).
'led' written backwards gives 'del'.
'del'+'ta'='DELTA'
(Other definitions for delta that I've seen before include "Type of wing" , "Dealt (anag.)" , "Split river mouth system" , "sort of wing" , "Area of distributaries in a river basin" .)